AIKIDO-2026-663412

TL;DR

Prefect validates user-supplied JSON schemas with the jsonschema library, which resolves $ref references through a default registry that fetches external URLs over HTTP during instance validation. A schema containing a remote $ref that points at an internal host or a link-local cloud metadata address causes the server to issue a blind outbound request while validating, and the failure surfaces as an uncaught referencing error rather than a clean validation error. This lets a caller probe internal services and metadata endpoints reachable from the server. The fix adds a non-fetching registry that resolves only in-document references and raises a controlled error for external refs without performing any network request.